A system of the type mentioned at the beginning is known, for example, from DE 20 2007 018 500 U1. In the known system, the adapter piece isolates the clamp, used there as a spring element, from the rail. For this purpose, the adapter piece is manufactured from an electrically non-conductive material. In order to allow a particularly simple assembly at the same time, the adapter piece in the known system has a holder, which is adapted to the shape of the end section allocated to the isolating clip such that the spring section can be inserted into it in form fit. The end section of the spring element concerned can therefore be inserted into the holder such that at least one segment of its periphery is surrounded by material of the adapter piece. In addition, the known adapter piece has a contact surface extending under the holder. This contact surface guarantees that the adapter piece is always guided such that the end section of the spring element engaging in its respective holder constantly remains in the holder concerned. In this way, the adapter piece can easily be moved from a pre-assembled position on the guide plate of the known system onto the rail foot, without manual intervention of a fitter or particular measures by an assembly machine being required in addition.
However, under the rough conditions which exist in practice, unintended changes in the positions of the spring element and the adapter repeatedly occur. These may be large enough to hamper the full assembly of the system from the pre-assembly state.